In recent years, so-called water-absorbent polymers are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent polymers to absorb body fluids.
Known examples of the above water-absorbent polymers include: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; and hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked products. However, almost all of these conventionally known water-absorbent polymers are products by neutralizing their acid groups with compounds of alkaline metals. Therefore, in the case where the surface neighborhood of particles of the polymers is not secondarily crosslinked, their absorption capacity under a load for a physiological salt solution is below 10 g/g, and further, even if the surface neighborhood of particles of the polymers is secondarily crosslinked, their absorption capacity under a load for a physiological salt solution is merely 25 g/g at the highest, and is therefore still insufficient to the use for disposable diapers or sanitary articles. In addition, because those conventional water-absorbent polymers merely exhibit a remarkably low absorption capacity for aqueous liquids containing electrolytes, their application range is limited. Thus, it is desired to develop a novel salt-water-absorbing agent which can rapidly absorb salt water to swell therewith even under loaded conditions.
To solve such problems, WO 96/15162, WO 96/15180, WO 96/17681, WO 98/24832, and WO 98/37149 propose water-absorbing agents which comprise water-swellable crosslinked polymers of acidity in water and compounds of basicity in water and utilize the ion-exchanging function and desalting mechanism due to interactions between the acid and the base. However, processes as described in these patents have problems in that: because the compound of basicity in water and the water-swellable crosslinked polymer of acidity in water are allowed to exist separately from each other, the saltwater absorption speed under a load is slow, and further, the absorption capacity under a load is low.